A son was also born to Seth, and [Seth] named him Enosh. It was then initiated to pray with God's name.

Commentary:

knew Mentioned after they ate from the Tree of Knowledge. Rashi notes that Cain and Abel were born before the sin.

gained Literally 'bought.' In Hebrew this is kanah, from which the name Cain (Kayin in Hebrew) is derived.

AbelHevel in Hebrew, literally a breath or vanity. He was called this because he never lived to have children (Midrash HaGadol; cf. Ramban).

An era ended Literally, 'It was the end of days.' It is significant that this same expression is used to denote the Messianic era, when the present era will end. It possibly refers to the expulsion from Eden, whereupon a new era began, see note on Genesis 4:1. Most sources render this, 'in the course of time.'

Naamah According to some traditions, she was Noah's wife. In the Midrash, however, this is disputed, and other ancient sources state that Noah married his niece, Amzarach, daughter of Rakh'el (Sefer HaYov'loth 4:33). (Regarding Sefer HaYov'loth, see Bibliography.)

I have killed... According to traditions, Lemekh had just killed Cain and Tuval Cain.

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